In Deinococcus radiodurans and other resistant bacteria, a mechanistic link exists between resistance, manganese accumulation, and protein protection. We have now shown that ultrafiltered, protein-free preparations of D. radiodurans cell extracts prevent protein oxidation at massive doses of ionizing radiation. In contrast, ultrafiltrates from ionizing radiation-sensitive bacteria were not protective. The Deinococcus radiodurans ultrafiltrate was enriched in manganese, phosphate, nucleosides and bases, and peptides. When reconstituted in vitro at concentrations approximating those in the Deinococcus. radiodurans cytosol, peptides interacted synergistically with manganese and preserved the activity of large, multimeric enzymes exposed to 50,000 Gy, conditions which completely destroyed DNA.